1. Field of the Invention
This invention relates to semiconductor fabrication processes, and more particularly, to a method for forming shallow trenches in a semiconductor substrate with rounded top corners that can be shaped with a desired radius of curvature.
2. Description of Related Art
In semiconductor technology, the so-called isolation process is used to suitably isolate the transistor elements in an integrated circuit (IC) chip from each other so as to prevent interference or a short-circuit therebetween. Conventional isolation processes are usually based on the so-called local oxidation of silicon (LOCOS) method. In more advanced VLSI (very large-scale integration) technology, hundreds of thousands transistor elements can be integrated on a single IC chip with an area of less than 2 cm.sup.2. To provide isolation for such a great density of transistor elements in the IC chip, the so-called shallow trench isolation method is used in place of the LOCOS method. In a shallow trench isolation process, the anisotropic dry etching process is performed on the substrate so as to dig trenches between neighboring transistor elements in the substrate. After that, an insulating material, such as silicon dioxide, is filled into the trenches to isolate the neighboring transistor elements.
If the top corners of the thus-formed trenches are too steep, this will cause a concentrated stress that increases the electric field on the gate, thus causing the occurrence of leakage current in the IC chip. This adverse consequence is the so-called corners effect. To provide a rounded top corner for the trenches, a conventional solution is to form a beak-like oxide layer on the substrate and then remove the beak-like oxide layer by etching. This conventional method will be described in detail in the following with reference to FIGS. 1A-1B.
Referring first to FIG. 1A, a semiconductor substrate 100 is prepared. Over the substrate 100, a pad oxide layer 102, such as a layer of silicon dioxide, is formed. Further, a mask layer 104, such as a layer of silicon nitride, is formed over the pad oxide layer 102. A photolithographic and etching process is then performed on the substrate so as to remove selected portions of the mask layer 104 and pad oxide layer 102 to form an opening 101 which exposes a selected surface area of the substrate 100.
Referring next to FIG. 1B, in the subsequent step, a thermal oxidation process is performed on the substrate so as to form a beak-like oxide layer (not shown) on the exposed surface area of the substrate 100. After this, the beak-like oxide layer (not shown) is removed by etching. The empty space which was previously occupied by the removed oxide layer serves as a shallow trench 103 with round top corners 103a.
One major drawback to the foregoing method, however, is that the round top corners 103a of the shallow trench 103 is shaped with a substantial radius of curvature of about 0.0587 .mu.m (micrometer), which can not be arbitrarily shaped in a predetermined manner into a desired radius of curvature in accordance with various requirements.